We have previously reported that the fractional synthesis rate of vastus lateralis mixed muscle protein (MMP) is lower in elderly than in young adults, although the effect of age upon the synthesis rate of individual contractile proteins was not examined. We used a constant intravenous infusion of [13C]leucine to determine the in vivo rate of amino acid incorporation into vastus lateralis MMP, myosin heavy chain (MHC), and actin in 23-30 yr old healthy, sedentary women and men (n=6) and in 78-84 yr old healthy, sedentary men and women (n=4). In general, lower MMP, MHC, and actin synthesis rates were found in the elderly, although the relative differences in the synthesis rates of individual proteins were not proportional between the two age groups. These results suggest that age-related decrements in contractile protein synthesis rates do not occur uniformly across these muscle proteins.